(Al)GaN-based high electron mobility transistors (HEMTs) are needed as the next generation of high power devices which could transform the current US electrical grid into a more stable and reliable power distribution network. GaN- and AlGaN-based HEMTs are excellent candidates for these high-power applications due to their material properties such as high electron saturation velocity, thermal conductivity, wide bandgap (WBG), and high critical electric field. Furthermore, in order to increase the capability for high power applications requiring several kV, there has been enhanced interest in pursuing materials systems with higher critical electric field and lower on-state resistance, such as ultra-wide bandgap (UWBG) materials with bandgaps approaching 6 eV. See A. G. Baca et al., “An AlN/Al0.85Ga0.15N high electron mobility transistor,” Appl. Phys. Lett., Vol. 109, PP. 033509-1 to 033509-4 (2016), which is incorporated herein by reference.
It has been postulated that there are two main routes for low resistance Ohmic contact formation for (Al)GaN: low Schottky barrier contacts and tunneling contacts. See M. E. Lin et al., “Low resistance ohmic contacts on wide band-gap GaN,” Appl. Phys. Lett., Vol. 64, No. 8, PP. 1003 to 1005 (1994), which is incorporated herein by reference. The low Schottky barrier contacts can be formed during high temperature anneals of the Ohmic metallization, in which the interfacial reaction between the metals and the underlying semiconductor results in reaction islands, or spikes, driving down to the heterostructure interface resulting in direct contact with the two-dimensional electron gas (2DEG) layer. See F. M. Mohammed and L. Wang, “Ohmic contact formation mechanism of Ta/Al/Mo/Au and Ti/Al/Mo/Au metallizations on AlGaN/GaN HEMTS,” J. Vac. Sci. Tech. B, Vol. 23, No. 6, PP. 2330 to 2335 (2005); F. M. Mohammed et al., “First-layer Si metallizations for thermally stable and smooth Ohmic contacts for AlGaN/GaN high electron mobility transistors,” J. Vac. Sci. Tech. B, Vol. 25, No. 2, PP. 324 to 333 (2007); A. Fontserè et al., “Micro and nano analysis of 0.2 Ωmm Ti/Al/Ni/Au ohmic contact to AlGaN/GaN,” Appl. Phys. Lett., Vol. 99, PP. 213504-1 to 213504-3 (2011); each of which is incorporated herein by reference. This reaction results in a very uncontrolled method for making direct metal contact to the 2DEG layer, with Fontserè et al. estimating only 5% of the total metal area is in contact with the 2DEG layer. See A. Fontserè et al., Appl. Phys. Lett., Vol. 99, at 213504-3.
Decreased contact resistance with a recessed Ohmic metallization has been previously reported for AlGaN/GaN heterostructures. See L. Wang et al., “Direct contact mechanism of Ohmic metallization to AlGaN/GaN heterostructures via Ohmic area recess etching,” Appl. Phys. Lett., Vol. 95, PP. 172107-1 to 172107-3 (2009), which is incorporated herein by reference. Wang et al. suggest that the lower contact resistance is due to direct electron conduction to the 2DEG layer at the edges, i.e., periphery, of an Ohmic contact pad.
In spite of these previous efforts, the need still exists for contacts having low resistance, low on-state resistance, and high current density for various devices made from wide and ultra-wide bandgap materials.